The stainless steel pipe production process which comprises producing stainless steel pipes by carrying out the steps of piercing rolling, elongating rolling using a mandrel bar, for example mandrel mill rolling, and sizing rolling and, further, subjecting the thus-obtained pipes, as mother pipes, to cold working is widely applied. In the following, such production process is explained in connection with the case of applying mandrel mill rolling as elongating rolling and stretch reducer rolling as sizing rolling.
A round steel block (billet) is heated to a predetermined temperature (generally 1150-1250° C.) using a heating furnace, such as a rotary hearth type, and this billet is passed through an inclined roll type piercing/rolling machine for making a hollow shell. Then, a mandrel bar coated with a lubricant is inserted into the hollow shell and the hollow shell is subjected to a single-pass rolling on a mandrel mill composed of 7 to 9 stands for roughening rolling to give a finishing rolling blank pipe with predetermined dimensions.
After this roughening rolling, the blank pipe to be subjected to finishing rolling is fed to a reheating furnace and reheated (generally to 900-1000° C.), the pipe outer surface alone is descaled by injecting high-pressure water jet, and the blank pipe is passed through a stretch reducer rolling mill to give a hot-finished pipe. When a cold working step follows, the pipe is referred to as a cold working mother pipe.
In the above-mentioned process of rolling the hot-finished pipe or cold working mother pipe, the mandrel bar to be used in the step of roughening rolling on a mandrel mill is inserted into the hollow shell in a high-temperature condition (generally 1100-1200° C.), creating the chance of readily causing seizure onto the hollow shell. The pipe profile and wall thickness after mandrel mill rolling is influenced by the roll revolving speed and roll caliber profile in the rolling step and further by the friction between the mandrel bar and the hollow shell.
Therefore, for preventing the seizure of the mandrel bar onto the hollow shell and for making the friction with the hollow shell proper so as to obtain the desired pipe profile and wall thickness, a lubricant is applied to the outer surface of the mandrel bar.
Known as such lubricant is, for example, a water-soluble lubricant based on graphite, which is inexpensive and has very good lubricating properties, as described in Japanese Patent Publication No. 59-37317, and this graphite-based lubricant has so far been used frequently. However, when a stainless steel material containing 10-30% Cr by mass is used, roughening rolling using a mandrel bar coated with a graphite-based lubricant incurs the phenomenon of carburization during rolling and a carburized layer having a higher carbon concentration than that of the base material is formed on the pipe inner surface side.
During the subsequent steps of reheating and rolling on a stretch reducer and further during the heat treatment steps, namely in the annealing heat treatment of the mother pipe, which is carried out prior to cold working, and the solution treatment, which is carried out in the final step, the carbon concentration in the carburized layer generated in the pipe inner surface decreases as a result of diffusion of carbon into the base material; however, the depth of the carburized layer increases and a carburized layer having a high carbon concentration still remains.
The main cause of the formation of a carburized layer in the pipe inner surface is the ingress of CO gas into the inside of steel, the CO gas being formed by gasification of part of graphite which is the main component of the inner surface lubricant, and/or part of carbon in the organic binder used therein, during mandrel mill rolling. As a result, the carbon concentration in the portion spanning about 0.5 mm deep from the surface in a thickness-wise direction sometimes becomes higher by about 0.1% by mass than that of the base material, so that it may exceed the upper limit of C content specified in Standard or the like in some cases.
In the carburized layer remaining with the level exceeding the specified limit, Cr, which is the main element forming a passivation film, namely an anticorrosive film, in stainless steel, is immobilized in the form of carbides, so that the corrosion resistance of the pipe inner surface is markedly deteriorated.
Therefore, those seamless stainless steel pipes which were subjected to the formation of a carburized layer in the pipe inner surface, cannot be shipped as products in as-is condition, so that measures for diminishing the carburized layer are taken. For example, the pipe inner surface where a carburized layer remains is wholly polished or, in Japanese Patent Application Publication No. 09-201604, a special heat treatment method is proposed which comprises subjecting the pipe after finishing rolling to descaling so as to reduce the thickness of the oxidized scale layer in the pipe inner surface and then keep the same for 3-20 minutes in an oxidizing atmosphere at 1050-1250° C. for decarburization. However, these methods of causing the carburized layer portion to disappear have a problem in that enormous man-hours and considerable costs are required for the treatment.
Further, in Japanese Patent Application Publication No. 08-90043, a process for producing seamless stainless steel pipes is proposed in which the mandrel mill rolling step is applied using a graphite-based lubricant, comprising reheating the finishing rolling blank pipe after mandrel mill rolling, in which the blank pipe whose inside is filled with an atmosphere containing steam in an amount of not less than 10% by volume is reheated and then finishing-rolled and, thereafter, further subjected to solution heat treatment. However, the production process proposed in the above-cited publication requires a fairly large-scale steam production apparatus for continuously passing steam of 10% by volume or more through the pipe inside.
Further, Japanese Patent Application Publication No. 04-168221 proposes a process for producing austenitic stainless steel pipes which comprises subjecting a finishing rolling blank pipe as obtained by mandrel rolling using a graphite-based lubricant to finishing rolling after 10-30 minutes of retention thereof in an atmosphere having an oxygen concentration of 6-15% in a temperature range of 950-1200° C. However, the production process proposed in the above-cited publication is impracticable from the yield viewpoint since the scale loss is great due to a long period of time required for heating the finishing rolling blank pipe.
And, in Japanese Patent Application Publication No. 08-57505, a process for producing austenitic stainless steel pipes which comprises replacing the atmosphere gas inside the blank pipe, after hollow shell rolling on a mandrel mill using a graphite-based lubricant, with an oxidizing gas prior to feeding it into a reheating furnace and feeding the oxidizing gas into the hollow shell inside during heating in the furnace.
The production processes proposed in the above-cited Japanese Patent Application Publication No. 08-90043, 04-168221 and 08-57505 all attempt to inhibit pipe inner surface carburization by subjecting the blank pipe to finishing rolling, such as stretch reducer rolling, after mandrel mill rolling using a graphite-based lubricant, and to apply decarburization treatment in reheating; the use of a graphite-based lubricant, however, still leads to a large extent of carburization.
Therefore, the effect of decarburization by feeding an oxidizing gas is restricted. For more reliable decarburization, it is necessary to raise the treatment temperature and prolong the treatment time, which produces the problem of scale formation and the resulting decrease in yield. Further, in all the production processes, no attempts have been made to improve the step of further cold working of the finishing-rolled mother pipe.
Therefore, recently, positive efforts have been made for the development of graphite-free lubricants and methods of using the same, in replacement of the above graphite-based lubricant, and Japanese Patent Application Publication No. 09-78080, for instance, discloses a lubricant which comprises, as main ingredients, layered oxides, namely mica, and a borate salt and is completely free of carbon or, if any, contains only the carbon in an organic binder component and thus has a carbon content lowered as far as possible.
The method of applying this graphite-free lubricant is the same as in the case of graphite-based lubricants, and the composition of the lubricant is designed so that the lubricant performance thereof may be equal to that of graphite-based lubricants. Thus, the graphite-free lubricant disclosed in Japanese Patent Application Publication No. 09-78080, when used properly, can prevent the carburized layer formation in the pipe inner surface.
On the actual premises operation, however, the mandrel bar surface is often contaminated with graphite.
Graphite-free lubricants are more expensive than graphite-based lubricants. Therefore, in the case of production of carbon steel pipes or low alloy steel pipes by elongating rolling using a mandrel bar, for example mandrel mill rolling, where no carburized layer is formed in the inner surface or a carburized layer, if formed, will not cause any particular problem, graphite-based lubricants are used from the economical viewpoint.
As a result, when a mandrel bar that has been used in elongating rolling of carbon steel pipes or low alloy steel pipes is used in producing stainless steel pipes, graphite inevitably remains adhering to the surface of that mandrel bar.
The graphite applied to the mandrel bar surface in elongating rolling of carbon steel pipes or low alloy steel pipes is spread abundantly on the mandrel bar transfer line, in particular the transfer line between the lubricant application area and the area of mandrel bar insertion into the hollow shell.
Therefore, even when a graphite-free lubricant is applied to the surface of the mandrel bar for using the same in elongating rolling of stainless steel pipes, the surface thereof (namely, the surface of the graphite-free lubricant film) is partly contaminated with the graphite already spread on the transfer line, irrespective of whether the mandrel bar has been submitted to elongating rolling of carbon steel pipes or low alloy steel pipes or not.
This graphite partly adhering to the graphite-free lubricant film surface comes into direct contact with the workpiece, namely the hollow shell; this causes the formation of a partially carburized layer in the pipe inner surface after rolling. Thus, the formation of a carburized layer is caused although there is a difference in extent as compared with the case of using a graphite-based lubricant.
On the other hand, in cases where a mandrel bar submitted to elongating rolling of carbon steel pipes or low alloy steel pipes is used, graphite remains adhering thereto beneath the graphite-free lubricant film newly applied and, as a result of severe working on an elongating rolling mill, the graphite remaining beneath the film also occasionally comes into direct contact with the workpiece and causes the formation of a partial carburized layer in the pipe inner surface during rolling and in the subsequent steps.
In this way, even when a graphite-free lubricant is used in elongating rolling using a mandrel bar, a carburized layer is formed in the pipe inner surface, and the carburized layer is selectively corroded in the descaling step comprising pickling of hot-finished pipes or pickling prior to cold working, resulting in surface roughening. The roughened surface caused by pickling remains, for example, in the form of pipe inner surface streak flaws even after cold working, thus deteriorating the surface quality.